The microsomal cytochrome P-450s have broad substrate specificities and metabolize exogenous materials such as carcinogens and drugs as well as endogenous agents such as cholesterol, prostaglandins, leukotriene, steroid hormones and fatty acids. By expressing various rat and human P-450 cDNAs in a variety of cell lines and characterizing the expressed P-450s, their material substrates and products, most of which have not been identified, could be determined. This furthermore provides another approach to study the structure-function relationship of each P-450 and its mode of action in vivo. We have inserted 10 forms of P-450 cDNAs obtained from rat and human tissue into the vaccinia virus-T7 expression vector. One of the P-450 cDNAs inserted into the vector, P-450LA, was expressed with a high efficiency in a variety of mammalian cell lines and possessed the catalytic activity towards 15- and 16-hydroxylation of palmitic acid. The expressed P-450 was also compared with the protein purified from rat liver microsomes. This P-450 was found to be constitutively expressed in mouse and rat hepatoma cell lines and inducible in the latter cell line by the hyperlipodemic drug, clofibrate. Other forms of P-450 will be expressed similarly in this vaccinia virus system.